The process of cutting continuous reinforcement fibers into fiber segments of discrete length is useful in the manufacture of different types of reinforcement structures. For example, the discrete length segments of reinforcement fibers can be used in reinforcement mats such as mats made with commingled fibers (e.g., glass fibers commingled with thermoplastic fibers), or laminated mats made from layers of fibers.
The discrete length segments of reinforcement fibers can also be used in reinforcement preforms. Structural composites and other reinforced molded articles are commonly made by resin transfer molding and structural resin injection molding. These molding processes have been made more efficient by preforming the reinforcement fibers into a reinforcement preform which is the approximate shape and size of the molded article, and then inserting the reinforcement preform into the mold.
To be acceptable for production at an industrial level, a fast preforming process is required. In the manufacture of preforms, a common practice is to supply a continuous length of reinforcement strand or fiber to a reinforcement dispenser or “chopper”, which cuts the continuous fiber into many fiber segments of discrete length, and deposits the fiber segments onto a collection surface. This process can be used to make preforms in an automated manner by mounting the reinforcement dispenser for movement over the collection surface, and programming the movement of the dispenser to apply the fiber segments in a predetermined, desired pattern.
The reinforcement dispenser can be robotized or automated, and such reinforcement dispensers are known art for such uses as making preforms for large structural parts, as in the auto industry, for example. (Dispensers of reinforcement fibers for the manufacture of mats of commingled fibers or laminated mats can also be adapted to be moveable and programmable.) Typically, the deposited fibers are dusted with a powdered binder, and compressed with a second perforated mold. Hot air and pressure sets the binder, producing a preform of reinforcement fibers which can be stored and shipped to the ultimate molding customer which applies resin to the preform and molds the resinated preform to make a reinforced product, typically using a resin injection process.
As the technical requirements for reinforcement structures increase, new methods for dispensing and laying down reinforcement fibers are required. One requirement is that the reinforcement fibers be delivered at faster speeds than used previously. Another requirement is that the reinforcement fibers be laid down in a predetermined orientation. The advancement in the reinforcement technology enabling a moveable and programmable reinforcement dispenser has led to requirements for very sophisticated fiber patterns and orientations. Reinforcement structures can be designed with specific amounts and orientations of reinforcement fibers to improve the strength of the structure precisely at the weakest or most stressed location of the article to be reinforced. Because of this new sophistication, there often is a requirement that the fibers be laid onto the collecting surface in a closely spaced, parallel arrangement.
U.S. Pat. No. 6,038,949 discloses a state of the art chopping device and method that generally provides the best performance to date. The device forms a strand into a loop that is fed along a form and generally flattened before being cut with rotating knives into individual fiber segments of desired length. While the apparatus and method disclosed in the U.S. Pat. No. 6,038,949 patent generally provide good performance, they suffer from a number of shortcomings and, accordingly, a need exists for an improved chopping device and method. More specifically, when processing a fiber material of a type comprising comingled unidirectional thermoplastic and glass fibers the device disclosed in U.S. Pat. No. 6,038,949 crutches the glass fibers and cuts the thermoplastic fiber. The hard and abrasive glass fiber rapidly wears the rotating knives which dull and then cannot cut the thermoplastic fibers. As a consequence, the knives must often be replaced thereby reducing productivity. In addition, it should be appreciated that the rotating knives have a fairly large diameter and must be placed at least one radius of the knife from the end of the chopping device.
Thus, the chopped fiber segments must be conveyed a significant distance along the device before they can be dispensed. Chopped fibers are difficult to handle and on occasion one or more fiber segments are dislocated, potentially resulting in the fiber being dispensed in an undesired orientation or position.
The present invention relates to an improved chopping device and method that utilizes grinding wheels to cut the fiber. Such grinding wheels have a longer service life than the rotating blades used in the prior art chopper and, accordingly, the present invention reduces maintenance down time and increases productivity. Further, the grinding wheels are positioned adjacent the discharge end of the chopping device so that the individual chopped fiber segments are only handled/conveyed for a very short distance before being dispensed. This substantially reduces the potential for dislocation of the fiber segments and thereby ensures proper, ordered handling of the chopped fiber segments and dispensing in the desired position and orientation.